Generally, a spark plug used for providing ignition in an internal combustion engine, such as an automotive engine, includes a tubular metallic shell; a tubular insulator disposed in a bore of the metallic shell; a center electrode disposed in a forward end portion of a bore of the insulator; and a ground electrode whose one end is joined to the forward end of the metallic shell and whose other end forms a spark discharge gap in cooperation with the center electrode. The spark plug generates spark discharges across the spark discharge gap formed between the forward end of the center electrode and the distal end of the ground electrode within a combustion chamber of the internal combustion engine, thereby combusting fuel which fills the combustion chamber.
In recent years, there has been developed a technique for increasing driving distance with less fuel through improvement of output by use of a supercharger. In such an internal combustion engine, temperature within a combustion chamber tends to increase; particularly, temperature in a region where the front end of the center electrode is located tends to increase. In order to cope with such temperature tendency, the following structure may be employed: a core higher in thermal conductivity than an Ni-based alloy; for example, a copper core, is disposed in an axial portion of the center electrode for promoting conductive release of discharge-generated heat (may be referred to as heat transfer) from the center electrode to the metallic shell.
However, in association with development of a small-sized spark plug for the purpose of enhancing the degree of design freedom for an internal combustion engine, the center electrode becomes slender. Such a slender center electrode may suffer the following phenomenon: in an environment of severe heating and cooling cycles, for example, the difference in thermal expansion between the copper core provided in an axial portion of the center electrode and an Ni-based alloy which surrounds the copper core causes plastic deformation or creep deformation of the center electrode, and the deformed center electrode pushes and cracks the adjacent insulator.
Patent Document 1 (Japanese Patent Application Laid-Open (kokai) No. 2009-170215) describes the invention whose object is “to provide a spark plug for an internal combustion engine configured such that, while a drop in thermal value is prevented, cracking of an insulator is restrained” (refer to Paragraph 0008 of Patent Document 1). In order to solve the problem, the described invention provides “a spark plug characterized in that: the center electrode has an engagement portion for determining the axial position of the center electrode relative to the insulator; the insulator has a seat portion formed on the inner wall of its axial bore for receiving the engagement portion of the center electrode inserted into the axial bore; and the inner wall of the axial bore has a parallel portion located forward of the seat portion and formed in parallel with the outer side surface of the center electrode, and a diameter-expanded portion formed between the parallel portion and the seat portion and being greater in clearance to the outer side surface of the center electrode than the parallel portion” (refer to claim 1 of Patent Document 1).
Patent Document 1 describes the following merit of the invention: when combustion residue enters a clearance between the center electrode and the insulator, the combustion residue preferentially accumulates in the clearance between the center electrode and the diameter-expanded portion of the insulator; thus, the combustion residue accumulated in the diameter-expanded portion serves as a stress relaxation layer to avoid direct transmission, to the insulator, of stress induced by thermal expansion of the center electrode, whereby cracking of the insulator can be restrained.
However, since the temperature within a combustion chamber tends to increase further, and the spark plug size tends to reduce, demand has been rising for a spark plug in which cracking of the insulator is further restrained.